The present invention is directed to an enhanced crude oil dehydration process and apparatus, and more particularly the present invention is directed to a crude oil dehydration process which can overcome the instability problems encountered with prior art for treating high water cut heavy oil streams, provide enhanced thermal energy input and recovery methods and remove suspended and dissolved compounds from inlet feed.
Throughout many regions of the world, heavy oil, a hydrocarbon material having much higher viscosity or lower API gravity (less than 20xc2x0 API, typically 7xc2x0 to 12xc2x0 API) than conventional petroleum crude, is being economically recovered for commercial sale. During the recovery process and prior to the transport to refineries for upgrading, the heavy oil receives preliminary treatment for water and solids removal to generally achieve basic sediment and water (BS and W) content less than 0.5% by volume and chloride content less than 30 ppm (wt). Water content of the treated heavy oil typically is required to be 0.3% by volume or less.
Conventional crude oil treatment methods were proven to be ineffective with respect to heavy oil until the advent of the technology set forth in U.S. Pat. Reissue No. 33,999, Clare et al., reissued Jul. 21, 1992 and Canadian Patent 1,302,937, Clare et al., reissued on Jun. 9, 1992. These patents describe a simple apparatus which can be located in remote oil producing areas for dehydrating heavy oil with low risk of foaming and unstable operating, while continuously achieving dry oil which exceeds requisite specifications. These dehydrators were found to be restricted to feed oil water content of less than 5% water cuts and susceptible to foaming and process instability during high water feed rates. Throughout the operation of several of these dehydrators known from practicing the technology in patents Re33,999 and U.S. Pat. No. 1,302,937, areas for improvement were discovered to overcome the limitations of feed oil water content and unstable operation caused by pretreatment upsets.
Further, additional problems have been experienced with the prior art in that although dehydrated heavy oil is achieved, high concentrations of suspended solids, such as clay and silica and dissolved compounds such as chlorides remain in the treated oil. These undesirable compounds continue to create many problems in pipeline transportation systems and refinery facilities to the extent that they depreciate the commercial valve of heavy oil.
Accordingly, one object of the present invention is to provide advances to overcome the limitations encountered by the previous art.
One object of the present invention is to provide for a dry crude oil recycle stream around the dehydrator to mix with the feed to reduce the ratio of residual water to oil contacting the oil surface of the dehydrator and thereby allow for higher raw crude oil water cuts, while maintaining a stable dehydration operation.
A further object of one embodiment of the present invention is to provide a method of removing water from crude oil containing water, comprising of steps of
a) providing a source of crude oil containing water;
b) providing a dehydrator for dehydrating the crude oil containing water, the dehydrator having an inlet and an outlet and a vaporizing surface of dry crude oil at temperatures sufficient to vaporize water contacting the surface;
c) exposing the source of crude oil to the dry crude;
d) vaporizing the water in the source; and
e) re-circulating at least a portion of dehydrated crude oil for contact with at least one of the dehydrator or the source of crude oil for maintaining a substantially uniform temperature at the vaporizing surface.
Conveniently, when at least a portion of the dry crude oil recycle stream around the dehydrator enters the dehydrator and is distributed below the surface of the hot crude oil in the dehydrator a consistent temperature is maintained at or above the vaporization temperature of water and at or below the surface of the oil and throughout the contained oil, thereby providing a means to mitigate the risk of process upsets and instability due to foaming.
A further object of the present invention is to provide a dry crude recycle stream around the dehydrator to mix with the feed stream, to allow an input of supplemental heat energy (external or waste heat energy) or recovery of heat energy from the dehydrator to result in an energetically efficient and balanced process.
A still further object of one embodiment of the present invention is to provide a method of removing water from crude oil containing water, comprising of steps of
a) providing a source of crude oil containing water;
b) providing a dehydrator for dehydrating the crude oil containing water, the dehydrator having an inlet and an outlet and a vaporizing surface of dry crude oil at a temperature sufficient to vaporize water contacting the surface;
c) contacting the source and the dry crude oil to flash water from the source to thereby remove the water from the source; and
d) selectively heating the surface of the dry crude to return heat energy lost from flash evaporating water from the source.
The dry crude oil surface may be selectively heated by reintroduction of dry crude oil, auxiliary heat addition, etc. The important aspect is that the heat used for vaporization is replaced so that a uniform or substantially uniform surface temperature is maintained. This is one important unit operation to maintain.
A further object of the present invention is to provide a means to remove suspended solids accumulated in the contained dry crude oil introduced with the source oil or produced during dehydration.
Suspended solids in the dry crude oil recycle stream, may be removed by a separator means on a continuous or batch basis to avoid buildup, plugging, and other complications.
A further object of the present invention is to water wash the raw crude oil source combined with the treatment in the dehydrator to remove the soluble dissolved solids or contaminants introduced the source crude oil and generate a low conductivity produced water and clean dry crude oil.
Until the advent of the present invention, prior art methods related to heavy oil dehydration had been limited by the stability of operation and risk of foaming, level of water cut or emulsion level in the heavy oil feed and the level of chloride, clay and silica compounds in the dry sales crude oil.
A still further object of one embodiment of the present invention is to provide a method of removing water from a crude oil containing water, comprising of steps of:
a) providing a source of crude oil containing water;
b) providing a dehydrator for dehydrating the crude oil containing water, the dehydrator having an inlet and an outlet and a vaporizing surface of dry crude oil at a temperature sufficient to vaporize water contacting the surface;
c) pre-washing the source of crude oil containing water to remove alkali metals and alkaline earth metals;
d) separating the alkali metals and the alkaline earth metals and oil in a preliminary separation step;
e) passing washed oil into the dehydrator to flash evaporate water from the washed oil to remove water therefrom;
f) selectively pre-heating crude oil to be treated in the dehydrator with treated dehydrated crude oil from the dehydrator to return heat energy lost from flash evaporating water; and
g) maintaining a uniform surface temperature.
Enhancements have been developed to eliminate the limits imposed by water cut of the source crude oil feed and to provide a very clean and dry heavy oil product relatively free of water, solids and chlorides.
The present invention relates to process enhancements to an apparatus used for dehydrating crude oil containing water, comprising a casing, means for admitting and distributing the liquid crude oil into the casing and onto the host surface of the dry crude oil, means for controlling the level of crude oil and a means to transfer heat energy sufficient to maintain the liquid oil at or above the distillation temperature for evaporating water and light hydrocarbons.
The light hydrocarbons and water exiting the casing are condensed by any suitable means known in the art, and collected and separated into water and light hydrocarbon liquid phases. Any non-condensible vapors are released from the apparatus for disposition by any safe means. Dry crude oil meeting pipeline BS and W specifications is pumped from the dehydrator for transport to refining and upgrading operations.
Typically, the dehydrator taught in the current art performed well to produce dry crude oil, however several problems have been encountered:
1. The dehydrator was limited to crude oil feed water cuts (wc) of less than 10% water to oil, and more specifically less than 5% wc to reduce the risk of unstable operation with foaming tendencies. This required the need for a conventional treater means upstream of the dehydrator to reduce raw crude oil water cuts from 50 to 20% wc down to less than 5% wc prior to feeding the dehydrator.
2. The dry crude oil exiting the dehydrator contains high chloride content, causing metallurgy and corrosion problems with downstream refineries facilities and transportation pipelines.
3. It was found that by flash evaporating off the water and by effectively eliminating all emulsions, solids such as clays and silica compounds, concentrated in the dry oil phase, had a tendency to buildup, plug and/or cause heat element damage.
4. It has been further experienced that the dehydrator is susceptible to unstable operations and foaming tendencies causing dehydration temperature swings and wet oil production.
The present invention seeks to address these concerns by providing methodology and apparatus to exceed the performance of the dehydrator beyond the prior art.
In one embodiment of the invention, at least a portion of the dry crude oil exiting the dehydrator is recycled and mixed with the inlet crude oil feed prior to entering the dehydrator casing. By increasing recycle flow, a consistent and stable inlet water cut composition can be maintained at the entrance to the casing to control the tendency to foam and create operational complications. With greater recycle rates, the raw water cut levels can be increased above the 10% wc stable level and continuous stable operation is maintained. This eliminates the need for conventional treatment ahead of the dehydrator and can avoid dehydrator process upset if an upstream treater is used and a treater upset occurs.
A further embodiment of the invention requires that at least a portion of the recycled dry crude oil be recycled and distributed immediately below the dry crude oil evaporating surface. This method ensures that the temperature of the surface of the dry oil in the dehydrator is maintained at or above the flash evaporating temperature of water. Water droplets from the feed are not permitted to penetrate the surface of the crude oil, thereby preventing the cooling below the surface and creating surface breakdown foaming and unstable dehydrator operation.
Advantageously, external heat transfer means can be added to the recycle circuit supra to regulate the precise temperature of the feed stream to the dehydrator casing. This method enhancement will regulate the precise level of pre-flashing of water vapour in the feed oil to control the residual water level contacting the hot dry oil surface. This step can be used to prevent the overcooling of the bath and eliminate the foaming effects caused by excessive evaporation surface breakdown.
The external heat transfer means can also be used to cool the feed stream. This may be used if the raw crude oil feed stream already contains sufficient or excessive thermal energy required to flash or distill the water vapor. Cooling the feed stream can regulate the flashing operation and prevent process complications.
As a further feature, a solid/liquid separation device, examples of which include a filter, hydro cyclone, centrifugal separators, gravity separators, centrifuge or any combination thereof, etc., may be employed in the circuit of the recycle stream continuously or on a batch basis to remove suspended solids from the hot dry oil.
Additionally, a clean water washing circuit may be added to the dehydrator feed to reduce undesirable dissolved compounds, such as chlorides, from the dry crude oil. The entire contaminated water stream, or a portion thereof, is treated by a suitable treatment method to create a clean water stream and a highly concentrated brine, slurry or solid product. The recovered clean water is recycled back to the raw crude oil for oil pretreatment. Generally water or any aqueous solution containing compounds for enhancing the extraction of chloride is most desirable, otherwise any regenerable fluid with a suitable aggressive solubility for chlorides may be considered.
It is preferable that in addition to achieving a dehydrated oil, having a BSandW content of less than 0.5% wc by volume, the embodiments of the invention in combination, or separately applied, can produce a dry clean crude oil, substantially free of solids, containing less than 30 ppm (wt) chlorides, in a continuous and stable operation, with low risk of foaming and process upsets. The oil produced by the present process is readily vendible and is most desirable, particularly in the case of heavy crude oils with gravities in the 7xc2x0 API to 20xc2x0 API range.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.